Then the deputy, when he saw what was done, believed, being astonished at the doctrine of the Lord. The deputy - believed - This was a proof that the doctrine was true; and that the power of God, from which nothing could be concealed, and which nothing could resist, was with these preachers.
Being astonished - Εκπλησσομενος, Being struck with astonishment, as Elymas was struck with blindness. Thus the word of God is a two-edged sword: it smites the sinner with judgment or compunction; and the sincere inquirer after truth, with conviction of its own worth and excellence.